Paradise Datacom Evolution, Инструкция по установке и эксплуатации, Страница: 240 / 354

Quantum and Evolution Series Installation and Operating Handbook
8-14
PCM channels (timeslots 1-15 and 16-31) and these are carried sequentially in pairs in
TS16 (there is no signalling channel for TS0 nor TS16, as TS0 is reserved for the Frame
Alignment Word, and TS16 carries the signalling for the other timeslots).
Robbed Bit Signalling (RBS)
Robbed bit signalling is a technique used on T1 PCM bearers whereby signalling is
conveyed by stealing the signalling bandwidth from the 64kbps timeslot to which it relates.
Once every 6 frames one bit (the least significant bit) of the 8 bits per timeslot is `stolen`
and used to convey signalling. In T1-D4 (which has a 12 frame terrestrial multiframe) this
gives two signalling bits `ab`, and in T1-ESF (which has a 24 frame terrestrial multiframe)
this gives four signalling bits `abcd`. The frames in which the bit is robbed are synchronised
to the terrestrial multiframe as follows: T1-D4: 6th & 12th, T1-ESF 6th, 12th, 18th, 24th.
Stealing the LSB every sixth frame was acceptable when PCM signals conveyed primarily
PCM audio as it made a negligible difference to the audio quality. However when the PCM
network was later used to convey data not just PCM encoded voice, only 56kbps of data
could be conveyed (using 7 bits per timeslot), as it was difficult to guarantee the signal
would not pass through a link using Robbed Bit Signalling where the 8th bit would be
overwritten every 6th frame, causing errors on the 64kbps data.
Signalling Over Satellite
Common Channel Signalling (CCS)
With Common Channel Signalling, the whole 64kbps signalling channel (timeslot) must be
conveyed from one end of the link to the other and typically this is done on sub primary
order satellite circuits (thin routes) by selecting the timeslot carrying the CCS for Drop/Insert
as with other data timeslots. Special action is required with CCS, as the equipment at both
ends of the link needs to be configured for the ` thin route` over the satellite so it does not
dynamically allocate traffic to a timeslot that is not passed over satellite link!
Channel Associated Signalling
The IBS specification includes an option for equipment to support CAS, and defines a
mechanism whereby the CAS (the 2kbps signalling channel per timeslot) can be carried
within the 6.7% IBS satellite overhead. This option is fully implemented within the modem
equipped with the Extended D/I feature and is further explained in the following section on
"CAS Multiframe".
Robbed Bit Signalling (RBS)
Historically PCM bearers using Robbed Bit Signalling could not be used with Drop/Insert,
as over standard IBS links the location of the signalling information (ie which timeslots have
a bit overwritten with signalling) was lost. Even with Timeslot ID Maintenance (which
preserved timeslot sequence and timeslot start / end boundaries) RBS was still lost. To
preserve RBS not only is TS ID maintenance required, but also the timeslots required
placing back into the correct frames within the terrestrial multiframe (so in T1-D4 for
example the timeslots containing the RBS are placed in frames 6 and 12 of the terrestrial
multiframe).
Intelsat’s IBS definition does not specify a mechanism to achieve this therefore the modem
equipped with the Extended D/I feature uses an extension of the Timeslot ID Maintenance
scheme to provide RBS support with Drop/Insert over `thin route` satellite links. It does this
by treating not just the timeslots from one frame as a group of `N` timeslots, but the group
from the whole terrestrial multiframe as a block within which timeslot identity has to be